1. Field of the Invention
The invention relates generally to the structure and fabrication of insulated gate field effect transistors, and more particularly to multi-layer metal gate electrodes.
2. Background
Advances in semiconductor manufacturing technology have led to the integration of tens, and more recently hundreds, of millions of circuit elements, such as transistors, on a single integrated circuit (IC). To achieve such dramatic increases in the density of circuit components has required semiconductor manufacturers to scale down the physical dimensions of the circuit elements, as well as the interconnection structures used to connect the circuit elements into functional circuitry.
One consequence of scaling down the physical dimensions of circuit elements has been a dramatic increase in the complexity of transistor engineering for the process engineers and scientists that develop advanced microelectronic manufacturing processes. In the past it was common to consider gate oxide thickness, polysilicon line width, source/drain junction depth and lateral diffusion therefrom, and some threshold adjusting ion implant as the primary parameters of transistor design. However as transistor dimensions entered the deep submicron region, the transistor structure became more complex with the inclusion of elements such as graded well doping, epitaxial wafers, halo implants, tip implants, lightly doped drain structures, multiple implants for source/drain formation, silicidation of gates and source/drains, and multiple sidewall spacers, among others. Because of the complex nature of deep submicron transistors, workers in this field tend to characterize, or specify, transistors not so much by physical dimension (e.g., gate length) but rather by electrical characteristics such as drive current (Ion) versus leakage current (Ioff).
For high speed operation of microelectronic components, circuit designers prefer transistors tuned for high drive currents. On the other hand, low leakage, i.e., low off-state current, is preferred so as to reduce power consumption. Typically, the structural and doping parameters that tend to provide the desired increase in drive current also tend to provide an undesired increase in leakage current.
What is needed are structures and methods for obtaining the desired electrical performance from insulated gate field effect transistors.
Briefly, insulated gate field effect transistors having gate electrodes with at least two layers of materials wherein the work function of the gate is substantially determined by thickness of one of the layers are disclosed. Such transistors provide gate electrode work function values that can be tuned for particular transistor performance requirements, and are similar to those of doped polysilicon, eliminate the poly depletion effect and also substantially prevent impurity diffusion into the gate dielectric.
In a further aspect of the present invention specific value ranges for gate electrode work function can be achieved by selecting particular thicknesses for a work function modulation layer that is disposed superjacent to a gate insulating layer.